The present invention relates to a culletless main sealing method of a cathode-ray tube, especially of a color picture tube of the type which does not produce a cullet during a sealing process.
FIG. 1 is an external view of a color picture tube after it is sealed enclosing an electron gun. The reference numeral 1 denotes a panel, 2 a funnel, and 3 a neck tube. The line A indicates an assumed datum line which is called a reference line, B a position of the under surface of a stem, and the distance l.sub.1 between A and B is determined from a point of view of a design.
FIG. 2 is an external view of a color picture tube before the sealing process of the type which produces a cullet after it is sealed enclosing an electron gun. The reference numeral 4 denotes a neck flare portion, and the neck tube 3 is cut by fusing at the portion C during the sealing process. The portion of the neck tube 3 including the neck flare portion 4 after it has been cut is a cullet.
FIG. 3 is an external view of a color picture tube before the sealing process of the type which does not produce a cullet and which is disclosed, for example, in Japanese Patent Laid-Open No. 128542/1981. This type of the tube is called a culletless tube, because the neck tube is cut at the position D in advance and thereby does not produce a cullet. The main sealing method of this type of color picture tube will be explained hereinunder.
This type of cathode ray tube is sealed enclosing an electron gun in the following way. The end surface of the neck tube is first softened by heating it with a gas burner. The stem preheated to a predetermined temperature is next pushed to the softened neck tube to be welded to the end portion of the neck tube. Thereafter, the stem is pulled and the welded portion is shaped so that the inner and outer surfaces of the welded portion are made even and have a uniform thickness. The stem here means a glass for fixing a lead, namely, the portion indicated by the reference numeral 8 in FIG. 8a which will be described later, and after it is welded to the neck tube, the portion is shown, for example, by the numeral 8 in FIG. 6a.
If the distance l.sub.2 between A and D in FIG. 3 varies, the positions of the end surface 5, with which the flames 6 of the gas burner for heating the end surface 5 of the neck tube 3 come into contact, also vary as shown in FIGS. 4a-4c. In an ordinary sealing equipment, the bulb is set with the panel 1 faced upward on the basis of the reference line A. FIG. 4a shows a normal case. In FIG. 4b the distance l.sub.2 is so short that the flames 6 come out of contact with the end surface 5, resulting in inadequate heating of the end surface 5. In FIG. 4c, the distance l.sub.2 is so long that the flames 6 heat the outer peripheral surface of the neck tube 3, thereby causing a difference in temperatures between the inner and outer surfaces of the neck tube 3, which disadvantageously makes the neck tube 3 easy to break and largely deforms it.
FIG. 5 shows an example of arrangement of burners 7, which are disposed apart from the plane containing the end surface 5 by the distance l.sub.3 such that the flames 6 come into contact with the end surface 5 of the neck tube 3 in an oblique direction. This method can cope with the variation of the distance l.sub.2, but the acceptable variation range is narrow. As a result, the bulb of a color picture tube is required to have a high precision, leading to an increase in cost.
Furthermore, with the variation of the distance l.sub.2, the volume of the welded portion between the heated end surface of the neck tube and the stem varies and, hence, the shape of the welded portion also varies when the stem is pulled in order to shape the welded portion after welding. FIGS. 6a and 6b are schematic sectional views of the stem and its vicinity after the completion of sealing. In FIG. 6a, the distance l.sub.2 is so short that some portion of the neck tube 3 in the vicinity of the stem 8 is extremely small in thickness (t), and if l.sub.2 is further shorter, a hole will be made between the neck tube 3 and the stem 8. On the other hand, in FIG. 6b, the distance l.sub.2 is so long that the welded portion is protruded outwardly from the neck tube 3, thereby making it impossible to attach a base thereto after the completion of evacuation. To prevent these troubles, the dimension l.sub.2 is required to have a high precision.